1. Field of the Invention
The present invention relates to medical ultrasound imaging and, in particular, to a medical ultrasound imaging system and method that provides dynamic focusing at high frequencies of operation and with high phase resolution.
2. Description of the Related Art
Medical ultrasound imaging systems are used to obtain information on the structural characteristics, such as shape and location, of human or animal tissue by applying an ultrasonic signal to the tissue and then analyzing the ultrasonic signal after it has interacted with the tissue to determine the various characteristics of the tissue. More specifically, operation of the typical medical ultrasonic imaging system includes the generation of an ultrasonic signal using an array of piezo-electric devices and applying the ultrasonic signal to the tissue of interest, such as the liver. As the ultrasonic signal propagates through the tissue, the tissue reflects the ultrasonic signal to varying degrees depending on the characteristics of the tissue. For example, denser tissue, such as a tumor, may reflect more of the ultrasonic signal than healthy tissue. The reflected ultrasonic signal is generally received by the same array of piezo-electric transducers that generated the ultrasonic signal and converted into a plurality of electrical signals that have substantially the same frequency as the received ultrasonic signal. These electrical signals are processed to recover the information on the tissue that they contain and to also place them in a form that can be displayed on a monitor or other output device for analysis by a physician or technician.
The processing of the plurality of electrical signals generally involves phase-shifting and/or time delaying the plurality of electrical signals with respect to one another to account for various operational parameters. For example, the phase-shifting and/or delaying of the electrical signals can account for, among other things, the angle of the wave front of the ultrasonic wave with respect to the array of piezo-electric transducers and the shape of ultrasonic wave. For instance, if the transmitted ultrasonic signal is a planar wave whose wave front is at an angle to the plane of a linear array of piezo-electric transducers, then the reflected signal produced by the tissue with which the wave front of the ultrasonic signal is in contact at any one moment in time will return to and come into contact with the elements of the piezo-electric array at different times. Since the reflected ultrasonic signal reaches the elements of the piezo-electric array at different times, the elements of the piezo-electric array produce electrical signals representative of the tissue that produced the reflected ultrasonic wave at different times. Consequently, to develop image data on the tissue structure with which the wave front of the ultrasonic signal is engaged at any one point in time, the electrical signals produced by the array of piezo-electric transducers must be appropriately phase-shifted and/or delayed in time with respect to one another. The process of phase-shifting and/or delaying the electrical signals with respect to one another is commonly referred to as "focusing and/or steering" the electrical signals and the structure for accomplishing the "focusing and/or steering" is commonly referred to as a beamformer. In some types of beamformers, phase-shifting is used to accomplish "fine" focusing that involves phase shifts of no more than 360.degree. or one cycle of the electrical signal and time delaying of the electrical signal is used to accomplish "coarse" focusing that is characterized by delays of one or more cycles of the electrical signal.
In addition to the "focusing" of electrical signals, the quality of the image data can also be affected by weighting the amplitudes of the various electrical signals with respect to one another to apodize or "shade" the image data. This can be useful in reducing sidelobes and improving dynamic range in the resulting image.
There is a wide variety of electronic system architectures used for beamforming. This includes systems that process the electronic signals simply by switching time delays in the paths of all the electronic signals. Others convert the analog signals to a digital format, using A/D converters, and then create the appropriate delays in shift registers or memories. Another type of beamformer employs a combination of heterodyning circuitry, to adjust phase, and time delaying circuitry. These beamformers have distinct disadvantages in size and cost when processing signals at higher operating frequencies, for example in the range of 2.5 to 15 MHz.
Presently, one of the most commonly employed analog-based beamformers employs heterodyning to accomplish phase-shifting of the electrical signals. Heterodyning of one of these electrical signals involves mixing or multiplying the electrical signal, which has a frequency substantially identical to that of the received ultrasonic signal, with a mixing signal of a different frequency to produce a signal with a frequency spectrum that is down-shifted from the frequency of the original electrical signal. The down-shifting allows componentry with reduced bandwidth or operating range requirements to be used. By switching between mixing signals of the same frequency but of a different phase, phase-shifting of the electrical signal to achieve "fine" focusing is also accomplished. Delaying of the electrical signals to achieve "coarse" focusing is typically accomplished with some sort of delay line structure.
In a medical ultrasound imaging system that employs heterodyning, amplitude weighting of the electrical signals to apodize or shade the signals produced by each of the piezo-electric transducers is accomplished by a different piece of circuitry from the mixer or heterodyne circuitry.
Presently, there are several applications for medical ultrasound imaging systems that current analog-based medical ultrasound imaging systems, and especially those that employ heterodyning, are not thought capable of addressing at a reasonable cost. Specifically, there is a need for a medical ultrasound imaging system that is capable of producing high quality image data from the electrical signals produced by a linear or curved array of transducers and by operating at a high frequency, typically in the range of ten megahertz or more, and doing so at a reasonable cost. The advantage of using a linear or curved transducer array is that the ultrasonic signal produced by the array can be electronically "steered" so that the mechanical "steering" required in, for example, annular arrays is avoided. Typical linear and curved transducer arrays require a large number of elements, tens to hundreds of elements, for adequate performance. Consequently, such systems must be capable of processing many electrical signals to produce an image. High frequency operation allows high quality data on small tissue structures or organs to be achieved. Specifically, there is an inverse relationship between the size of the tissue or organ of interest and the frequency of the signal that can be employed to obtain image data, due to the filtering characteristics of animal tissue. For example, if the tissue of interest is relatively small, then a higher frequency ultrasonic can be used. The use of a higher frequency signal, in turn, allows high quality image data to be realized. Presently, there is a need to provide image data on tissue structures that are relatively small, such as bile ducts, fallopian tubes, and small coronary arteries and veins. Unfortunately, there is significant expense involved in high frequency heterodyne systems in making the small phase adjustments in an electrical image signal that are necessary to realize high quality image data. This expense is further amplified in heterodyne systems that use multi-element linear or curved transducer arrays.
A further drawback associated with heterodyne based medical ultrasound imaging systems that operate at high frequencies and that switch from one mixing signal to another mixing signal to produce a phase-shifted signal is that switching transients which have significant frequency components at or near the frequency of the ultrasonic signal are produced. These switching transients can adversely affect the information contained in the electrical signals produced by the piezo-electric transducers. Moreover, elimination of the switching transients typically requires the implementation of sophisticated and costly filtering techniques.
Yet another drawback associated with medical ultrasound imaging systems that accomplish phase-shifting by switching between mixing signals of different phases is that only discrete or step-wise phase shifts are possible. For example, one known medical ultrasound imaging system provides phase-shifts in steps of 22.5.degree.. Due to this limitation, such systems exhibit limited phase resolution that adversely impacts image quality.
A high quality image also generally requires the ability to amplitude-weight the electrical signals to apodize or "shade" the electrical signals produced by each of the piezo-electric transducers in the array. Achieving apodization in heterodyne based medical ultrasound imaging systems typically requires the use of programmable attenuators, one for each electrical signal, that are also relatively expensive. Consequently, for heterodyne systems that use multi-element linear or curved transducer arrays, significant expense is typically in involved providing apodization capability.
Additionally, there is a need for a medical ultrasound imaging system that can compensate for changes in the center of frequency of the received ultrasonic signal. The center frequency of the received ultrasonic signal is the frequency around which much of the componentry in a beamformer is designed. Deviations from the center frequency adversely affect the performance of the beamformer and the resulting image data. Animal tissue exponentially attenuates the ultrasonic signals applied to it at a rate that is frequency dependent and, in so doing, shifts the center frequency of the received ultrasonic signals. Further, this shift in the center frequency increases as the ultrasonic signal propagates further into the tissue of interest. Consequently, the shift in the center frequency becomes especially troublesome when the imaging system operates at high frequencies. Compensation for shifts in the center frequency requires expensive circuitry in heterodyned systems, especially if high phase resolution is also required and the center frequency of several electrical signals must be adjusted, as is typically required when multi-element linear and curved transducer arrays are utilized.
There is yet a further need for a medical ultrasound imaging system that can be readily adapted to operate at different frequencies. As previously mentioned, there is an inverse relationship between the frequency of the ultrasonic signal that can be used and the size of the tissue or organ of interest. As a result, higher frequencies are appropriate for smaller organs or tissue and lower frequencies are more appropriate for larger organs or tissue. Based on this, it is desirable that a medical ultrasound imaging system be adaptable to operate at different frequencies so that organs or tissue of different sizes can be imaged.
Moreover, there is a need for a medical ultrasound imaging system that can compensate for systematic errors, i.e., errors in amplitude and phase that are attributable to the components that comprise the system. For instance, the delay line length may not be appropriate for the specific delay required of it and, as a result, adversely affect the quality of the resulting image data. In this case as well as in the cases of other sources of systematic error, it is desirable to be able to compensate for these errors to improve the quality of the resulting image data.
Further, there is a need for a medical ultrasound imaging system that can process signals that have interacted with tissue very close to the transducer array. Typically, such signals are so strong that processing circuitry is incapable of processing them in a reliable manner.
Further, there is a need for a medical ultrasound imaging system that reduces the number of parts to, in turn, reduce the part cost and assembly cost as well as improve the reliability of the resulting medical ultrasound imaging system.